Well, the economic slump means fewer people driving for entertainment purposes, and more fatal accidents occur when people are driving to and from entertainment. That's one explanation, according to Ray LaHood, in addition to safer vehicles.
From CNN: "Alcohol-related traffic deaths, one of the leading types of fatal traffic accidents, declined by 7.4% between 2008 and 2009, NHTSA said. Last year 10,839 people were killed in alcohol related crashes, the agency said, about a third of all traffic deaths."
Motorcycle accidents have also decreased significantly.
Thanks to W.A. Davis